Bipolar transistors are important components in, for example, logic circuits, communication systems, and microwave devices. One type of bipolar transistor is a silicon germanium (SiGe) heterojunction bipolar transistor (HBT). An SiGe HBT can typically handle signals of very high frequencies, e.g., up to several hundred GHz.
Strained SiGe is typically the film of choice for application in NPN HBTs. The SiGe is pseudomorphically grown to match the silicon lattice beneath the SiGe and is, therefore, in a compressively strained state. Subsequent to the pseudomorphic growth process (and in the same reactor) a cap layer (e.g., a silicon cap layer) can be grown. The silicon cap layer is conventionally doped n-type during the same process using either arsenic (As) or phosphorus (P)—e.g., arsine (AsH3) and phosphine (PH3) are typical dopant gases. The silicon cap layer maintains the SiGe in a strained condition during thermal anneal processes. Next to the silicon cap layer, a base-emitter heterojunction is typically formed within an SiGe HBT.
The base-emitter heterojunction within an NPN SiGe HBT results in a bandgap offset between the base and the emitter. The addition of germanium (Ge) to the bulk silicon lattice results in a bandgap reduction, which occurs mostly in the valence band. The mild valence bandgap offset also provides a potential barrier against hole diffusion from the base to the emitter. The combination of conduction band lowering and valence band lifting results in an increase in collector current and a reduction in base current and, consequently, a large increase in current gain. Such results permit an increase in base doping of an SiGE HBT to further reduce base resistance (RB) for an enhanced Fmax (Fmax α1/RB).
In addition to a large increase in lattice strain and the bandgap offset, the addition of germanium (Ge) to the silicon lattice of the base region of an SiGe HBT results in significant reduction in boron diffusion rates. Such a reduction permits for a narrower base width to reduce transit time and increase device speed of operation.
The requirement for a narrow boron doped (p-type) base region to achieve high transmit frequency (Ft) values results, however, in very high current gains along with greatly reduced breakdown voltages, especially the collector-to-emitter breakdown (BVCE0).
Accordingly, what is needed are methods of material engineering that will reduce current gains and increase the breakdown voltages, e.g., the BVCE0, of an HBT (e.g., an SiGe HBT) without adverse affect to device speed and power requirements. The present invention addresses such a need.